The present invention relates to a copier operable in a two-sided copy mode and a combination copy mode, as desired.
A copier capable of producing a two-sided copy carrying an image on both sides thereof, a copier capable of producing a combination copy carrying a plurality of combined images on one side thereof, and a copier having both of such capabilities are extensively used today. It is a common practice with this kind of copier to use an intermediate tray for temporarily stacking copy sheets each carrying an image on one side thereof. A feed roller refeeds such copy sheets one by one from the intermediate tray to an image transfer station where a photoconductive element is located. The copy sheets are stacked on the intermediate tray face up or face down, as the case may be. Usually, the copy sheets are stacked face up in a two-sided copy mode and face down in a combination copy mode. It has been customary to refeed the lowermost copy sheet first or the uppermost copy sheet first at all times when the feed roller is so configured. Such a scheme has some problems left unsolved, as follows. Assume that the copy sheets are stacked on the intermediate tray face down, i.e., with their sides carrying toner images facing downward. When the uppermost copy sheet is refed while sliding on the underlying copy sheet, the former has its image portion strongly rubbed against a portion of the latter which corresponds to the feed roller. As a result, a small amount of toner comes off the image surface of the copy sheet and rubbed against the portion of interest of the copy sheet, smearing the the copy sheet.
On the other hand, assume that the copy sheets are stacked on the intermediate tray face up, i.e., with their sides carrying the toner images facing upward. Then, the image in the portion of the underlying copy sheet corresponding to the feed roller rubs against the overlying copy sheet. Nevertheless, it is not that the image in the portion of interest of the copy sheet rubs itself against the same portion of the overlying copy sheet, but that the former rubs against the leading edge to the trailing edge of the latter, i.e., over the entire length of the latter. Hence, despite that some toner comes off the copy sheet and deposits on the copy sheet, it is scattered over the surface of the copy sheet and, therefore, does not noticeably contaminate the copy sheet.
For the same reason, when the copy sheets are stacked on the intermediate tray face up and the lowermost one of them is refed first, the toner smears the copy sheets. However, when the lowermost one of the copy sheets stacked face down on the tray is fed first, no noticeable contamination occurs.
As stated above, the copy sheets are smeared or not smeared, depending on the orientation of the image surfaces of the copy sheets and which of the copy sheets is refed first. To eliminate the contamination by the toner, it is preferable that the copy sheets be stacked on the intermediate tray face up and then refed therefrom, the uppermost copy sheet being first. However, although such a manner of refeed is feasible for a two-sided copy mode, it is not practicable when it comes to a combination copy mode. Specifically, in a combination copy mode, should such a refeed principle be used, the surface of each copy sheet opposite to the surface which carries an image and should meet another image to be combined at an image transfer station would meet the image of interest. It has been customary, therefore, to use two different intermediate trays, i.e., one for feeding the uppermost one of copy sheets stacked face up first, and the other for feeding the lowermost one of copy sheets stacked face down first. The first-mentioned intermediate tray is assigned to a two-sided copy mode while the second-mentioned intermediate tray is assigned to a combination copy mode. This kind of implementation is disclosed in, for example, Japanese Patent Laid-Open Publication No. 206742/1986. Although such an implementation successfully deals with both of a two-sided copy mode and a combination copy mode and prevents copy sheets from being smeared, it increases the overall dimensions and cost of the copier due to the two intermediate trays.